Men and women are very different, but all of these differences start with one small chromosome. The Y chromosome is what separates the males from the females, but have you ever wondered how it came to be, or how it creates such differences? These are questions that Professor Emeritus Jennifer Graves at La Trobe Institute of Molecular Science has researched at great length. She recently presented a seminar regarding many aspects of sex chromosomes, but Y evolution is of particular interest.
How is Sex Determined?
At a young age we learn that if you are a female you have two X chromosomes and males have an X and Y. This Y chromosome in humans is what contains the testis-determining factor that essentially “turns on” the testes to produce the hormones that make a male. This testis-determining factor is called the SRY (Sex-determining Region Y) gene and is what makes the X and Y chromosomes so different. Additionally the Y chromosome is also a lot smaller, with only 45 genes in comparison to the hundreds found on the X chromosome. Together these differences in the Y chromosome are how sex is determined between males and females in humans.
How did these differences in the X and Y chromosome come about? The evolution of the Y chromosome to its current status has been a process that has taken millions of years. Originally X and Y were an ordinary homologous pair of chromosomes, but they began to differ when one mutated and acquired the male determining gene SRY. This was the beginning of the Y chromosome. Over time, other genes advantageous to males accumulated making the pair even more different.
A consequence of this evolution led to X and Y not being able to align and recombine as shown in the figure above. As a result, The Y chromosome was slowly degraded to its present state. If the gene deletion and mutation of the Y continues it could result in such degradation as to completely lose function. Graves estimated that the Y chromosome could completely disappear in approximately 4.6 million years.
Is the Y chromosome disappearing?
Whether or not the Y chromosome is actually disappearing is a matter of some debate among researchers. Graves argues that due to the high variation in the Y, inefficient selection, and drift are continuing to degrade the Y. This has also been shown in other mammals and has actually disappeared in spiny rats. Other researchers have argued that the Y chromosome has been conserved for hundreds of millions of years and has not lost any genes in the last 6 million years. Although Graves believes the Y continues to degrade, she feels the actual disappearance is not likely with our large populations, which prevents drastic genetic drift. Additionally, if the disappearance will take around 4.6 million years it is likely we will not survive that long, especially when considering the overpopulation and overconsumption concerns I described in an earlier blog.
The Y chromosome disappearance is still a matter of debate, but future research will hopefully gain even further insights into Y evolution. Much of Graves research has been over a wide variety of species and she is currently working on gene mapping in the Tammar wallaby. By looking at how sex is determined over multiple species we can gain a greater understanding of our own genetics. I have only discussed much of the Y chromosome in humans, but there are many other interesting questions regarding sex determination with other species.
- The origin and function of the mammalian Y chromosome and Y‐borne genes–an evolving understanding; Graves, 1995
- Y-chromosome evolution: emerging insights into processes of Y-chromosome degeneration; Bachtrog, 2013
- Is the Y-chromosome disappearing? – Both sides of the argument; Griffin, 2012
- Sex-determination: Gamble & Zarkower, 2012
- The evolution of the sex chromosomes: Step by step
- Weird animal genomes and the evolution of vertebrate sex and sex chromosomes; Graves, 2008